This thesis deals with the application of the Life Cycle concept to the design of bridges. Its focus is on methodology rather than on any particular application, with emphasis on the preliminary design stage. With the help of conceptual equations for the construction, maintenance, and vehicle delay costs, a procedure for making decisions at the preliminary design stage is presented. Decisions are made on the basis of minimum present equivalent total cost and include the number of lanes to be provided, the span between piers, and the design alternative (type of bridge) to be adopted for detail design. Minimization is done by a total enumeration procedure. Sensitivity of the decisions with respect to the interest rate and the study period is analyzed, and design decision reversals are noted. This includes a joint sensitivity analysis with respect to these design independent parameters. Limited analysis of errors in cost estimating equations is also performed. A FORTRAN code, implementable on mainframe and personal computers is developed to aid calculations. / Master of Science